Quantum Chemistry with Dr Nicola Gaston
What we’re particularly interested in is illustrated by the version of the periodic table shown on this page. Here each metal atom in the periodic table is coloured according to the structure it adopts in the bulk, which is simply a way of describing the arrangement of the atoms in space. So aluminium, for example, has a fcc crystal structure which is a type of cubic crystal structure; this means that every atom in the bulk can be imagined to sit on the vertices of a cube. On the other hand, magnesium, which is next to aluminium in the periodic table (i.e. it has one less electron and one less proton in the nucleus) adopts a hexagonal structure (called hcp) where all the atoms are surrounded by six neighbouring atoms in stacked planes. Both these structures have the same density, and are referred to as close-packed – so why does a metal choose one of these structures over the other? We do know some of the reasons that metals adopt different structures; sometimes it is due to the influence of magnetism, such as in Mn, Fe, and Co. Sometimes it will be due to the increasing size of the atom, and sometimes due to the number of valence electrons, but on the whole, the biggest influence on the structure that is finally adopted is the detailed nature of the binding interactions. So a proper explanation of these structures really does require an accurate quantum mechanical description of the electrons. You could very well ask, that since we already know what the structures of these metals are: why do we need to predict them theoretically? Well, first of all, the structures shown are only the familiar structures known at standard conditions of temperature and pressure – cooling or compressing them can result in very drastic changes in structure, and sometimes even transitions between the insulating and metallic states. These changes of phase have huge implications for nanotechnology also – for example, nanoparticles of gallium do not adopt the structure that we observe for the bulk under standard conditions, but instead have structures that are seen for bulk gallium only at low temperature. These structural changes at the nanoscale are closely related to one of the main themes of nanotechnology; the importance of increased relative surface area. This has consequences for a whole range of important phenomena – for example, the increased catalytic activity of nanoparticles compared to macroscopic material is due to the increased proportion of atoms at the surface, and hence increased ability to catalyse the chemical reaction. The melting temperature of nanoparticles generally decreases with size, due to the larger relative surface area (or a higher proportion of atoms that are bound less strongly). So one of the things we are currently studying is why gallium nanoparticles, when they get small enough, start melting at temperatures that are higher than the bulk metal. Could this have something to do with the structures, which are so different from the structure of bulk gallium under standard conditions?